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51.
Bydder M Yokoo T Hamilton G Middleton MS Chavez AD Schwimmer JB Lavine JE Sirlin CB 《Magnetic resonance imaging》2008,26(3):347-359
Quantification of fat has been investigated using images acquired from multiple gradient echoes. The evolution of the signal with echo time and flip angle was measured in phantoms of known fat and water composition and in 21 research subjects with fatty liver. Data were compared to different models of the signal equation, in which each model makes different assumptions about the T1 and/or T2* relaxation effects. A range of T1, T2*, fat fraction and number of echoes was investigated to cover situations of relevance to clinical imaging. Results indicate that quantification is most accurate at low flip angles (to minimize T1 effects) with a small number of echoes (to minimize spectral broadening effects). At short echo times, the spectral broadening effects manifest as a short apparent T2 for the fat component. 相似文献
52.
Claude Bardos Uriel Frisch Walter Pauls Samriddhi Sankar Ray Edriss S. Titi 《Communications in Mathematical Physics》2010,293(2):519-543
We consider a modification of the three-dimensional Navier–Stokes equations and other hydrodynamical evolution equations with
space-periodic initial conditions in which the usual Laplacian of the dissipation operator is replaced by an operator whose
Fourier symbol grows exponentially as e|k|/kd{{{\rm e}^{|k|/k_{\rm d}}}} at high wavenumbers |k|. Using estimates in suitable classes of analytic functions, we show that the solutions with initially finite energy become
immediately entire in the space variables and that the Fourier coefficients decay faster than e-C(k/kd) ln(|k|/kd){{{\rm e}^{-C(k/k_{\rm d})\,{\rm ln}(|k|/k_{\rm d})}}} for any C < 1/(2 ln 2). The same result holds for the one-dimensional Burgers equation with exponential dissipation but can be improved:
heuristic arguments and very precise simulations, analyzed by the method of asymptotic extrapolation of van der Hoeven, indicate
that the leading-order asymptotics is precisely of the above form with C = C
* = 1/ ln 2. The same behavior with a universal constant C
* is conjectured for the Navier–Stokes equations with exponential dissipation in any space dimension. This universality prevents
the strong growth of intermittency in the far dissipation range which is obtained for ordinary Navier–Stokes turbulence. Possible
applications to improved spectral simulations are briefly discussed. 相似文献
53.
Sandrine Duluard Joseph Grondin Jean‐Luc Bruneel Guy Campet Marie‐Hlne Delville Jean‐Claude Lassgues 《Journal of Raman spectroscopy : JRS》2008,39(9):1189-1194
The Raman and Infrared (IR) spectra of poly(methyl methacrylate) (PMMA) membranes plasticized by ionic liquids of the (1 − x)[1‐butyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI)],xLiTFSI type, where BMI+ is the 1‐butyl‐3‐methylimidazolium cation and TFSI− the bis(trifluoromethanesulfonyl)imide anion, are analyzed for a lithium bis(trifluoromethane sulfone)imide (LiTFSI) mole fraction x = 0.23 and PMMA contents from 0 to 50 wt%. The lithium is found to have an average coordination of about three CO groups and less than one TFSI− anion. It plays the role of a cross‐linker between the ester groups of PMMA and the nonvolatile ionic liquid. Addition of PMMA to the (1 − x)(BMITFSI),xLiTFSI ionic liquid lowers the conductivity but might improve the lithium transference number by transforming the [Li(TFSI)2]− anionic clusters present in the pure ionic liquid into a mixed coordination by ester groups and TFSI− anions. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
54.
Aldée Cabana Yves Doucet Jean-Marc Garneau Claude Pépin Pascal Puget 《Journal of Molecular Spectroscopy》1982,96(2):342-350
The vibration-rotation bands 2ν1, 2ν3, ν1 + ν2, ν2 + ν3, and ν1 - ν2 of H12CP were recorded and analyzed. These data were combined with previously reported results for this molecule to obtain an improved and extended set of vibrational and rotational constants. All xij and γij were calculated except those that require data from a summation band involving ν1 and ν3. 相似文献
55.
We study the minimal spanning trees of a connected graph G = (X,U) where U is partially preordered (or quasi-ordered). We characterize several kinds of optimal spanning trees and give conditions for existence of strongly optimal trees. Generalizations to bases of matroids (binary matroïds in part 2) are immediate. Sone of our results are given in terms of Krugdahl's dependence graphs. They imply previous results of Rosenstiehl and Gale in the case of linear orders or preorders. 相似文献
56.
57.
Cron GO Wallace JC Stevens WD Fortin T Pappas BA Wilkins RC Kelcz F Santyr GE 《Magnetic resonance imaging》2005,23(5):619-627
The arterial input function (AIF) is important for quantitative MR imaging perfusion experiments employing Gd contrast agents. This study compared the accuracy of T(2)*-weighted magnitude and phase imaging for noninvasive measurement of the AIF in the rat aorta. Twenty-eight in vivo experiments were performed involving simultaneous arterial blood sampling and MR imaging following Gd injection. In vitro experiments were also performed to confirm the in vivo results. At 1.89 T and TE=3 ms, the relationship between changes in 1/T(2)* in blood (estimated from MR signal magnitude) and Gd concentration ([Gd]) was measured to be approximately 19 s(-1) mM(-1), while that between phase and [Gd] was approximately 0.19 rad mM(-1). Both of these values are consistent with previously published results. The in vivo phase data had approximately half as much scatter with respect to [Gd] than the in vivo magnitude data (r(2)=.34 vs. r(2)=.17, respectively). This is likely due to the fact that the estimated change in 1/T(2)* is more sensitive than the phase to a variety of factors such as partial volume effects and T(1) weighting. Therefore, this study indicates that phase imaging may be a preferred method for measuring the AIF in the rat aorta compared to T(2)*-weighted magnitude imaging. 相似文献
58.
Najate Bensari-zizi Claude Alamichel 《International Journal of Infrared and Millimeter Waves》1989,10(1):137-151
About 2500 lines of CH3
35Cl have been assigned. The strong xy Coriolis resonance between thev
2 andv
5 modes is quite visible between thev
4+v
4
±1
perpendicular band, centered around 4383 cm–1, and thev
4
±1
+v
5
±1
perpendicular component, centered around 4475 cm–1, with a crossing of upper energy levels allowing the observation of lines which are normally forbidden. Although not yet observed with certainty, because of the great density of lines of the spectrum, thev
4
1
+v
5
±1
parallel component is nevertheless detectable by its effects onv
2+v
4
±1
which is linked by Coriolis resonance to both components ofv
4+v
5. Moreover the spectrum is much complicated by many other resonances with weak bands which occur at level crossings: it is the case ofv
2+3v
6
±1
, connected tov
2+v
4
±1
by the well known Darling Dennison resonance which couplesv
4
±1
and 3v
6
±1
, and also ofv
5
±1
+3v
6
±1
connected tov
4
±1
+v
5
±1
by the same resonance; but this last case is complicated by an anharmonic resonance betweenv
5
±1
+3v
6
±1
and 2v
3+3v
6
1
. Two more perturbations occur on the K=–1 side ofv
2+v
4: a weak Coriolis resonance gives rise to one subband ofv
1+v
2 at a level crossing withv
2+v
4, and thev
1+v
5 band (linked of course tov
1+v
2 by the Coriolis resonance between thev
2 andv
5 modes) is quite visible and perturbs several subbands ofv
2+v
4 of high values of K through an anharmonic resonance. Moreover, the complex (3v
5
±1
,v+2v
5
0
, 2v
2+v
5
±1
, 3v
2,v
2+2v
5
±2
, 3v
5
±3
) system of Coriolis-connected bands is linked to the bands studied in the present work by two Fermi resonances: one betweenv
2+2v
5
0
andv
1+v
2, and the other one betweenv
1+v
5 and 3v
5
±1
, whose several subbands have been observed on the low part of the spectrum. The values of all the band centres and of the different coupling constants have been estimated, but all these interactions make the line assignments and the interpretation of the spectrum very difficult. 相似文献
59.
Ali Panahandeh Claude Oestges Jean-Michel Dricot François Horlin Philippe De Doncker 《Physical Communication》2012,5(4):317-327
Compared to classical spatially separated multiple antenna system, cross-polarized co-located antenna systems are an interesting way to reduce equipment size while reducing the inter-antenna correlation. In this paper the spectrum sensing of a Cognitive Radio (CR) system taking advantage of polarization diversity under Rayleigh fading is investigated and compared to an equivalent system using spatial diversity. This analysis is based on a theoretical formulation applied to a real-world scenario. For this purpose, an outdoor-to-indoor measurement campaign at a frequency of 3.5 GHz is realized, where an indoor secondary user senses the signals received from an outdoor primary base station. The signals received at each antenna are first combined and then applied to an energy detector. The theoretical expressions are simulated in the measurement context. The detection probability behavior as a function of distance between the Primary Transmitter (PTx) and the Secondary Terminal (STE) and the inter-antenna correlation effect on the sensing performance are studied. 相似文献
60.
Lévêque JM Desset S Suptil J Fachinger C Draye M Bonrath W Cravotto G 《Ultrasonics sonochemistry》2006,13(2):189-193
The replacement of common organic solvents by room-temperature ionic liquids (RTILs) is a topical subject in both academia and industry. In the last decades, the number of applications for RTILs has followed an exponential curve and spilled over the boundaries of chemistry. Still, one of the main drawbacks of these compounds is their difficult access. The present ultrasound-assisted method affords a general and easy access to a large variety of room-temperature ionic liquids. 相似文献